feat(ai): server-side chat history + assistant UX fixes

Server-side, per-user chat history (the thread now survives reload):
- new ai_chat_messages table + migration; getChatHistory/appendChatMessages/
  clearChatHistory; GET/POST/DELETE /api/admin/ai/history (ai.use-guarded);
  service + HTTP tests (isolation, ordering, clear, perm, validation).
- DashAssistant loads the thread on mount and persists each turn; "Vymazat"
  clears it. gcTime:0 so each mount re-reads the DB (no stale-cache resurrection);
  composer gated until history settles + seed flag flipped on submit (no
  seed/submit race clobbering a freshly-sent turn).

UX + correctness fixes from review/feedback:
- review cards moved OUT of the scrollable thread into a "Faktury k potvrzení"
  section so Uložit is always reachable; chat auto-scroll no longer fires on
  field edits.
- chat bubble text colour set on the Typography (GlobalStyles pins `p` to
  text.secondary, which was overriding the inherited bubble colour → unreadable).
- never clear input/staged PDFs on a failed submit (rollback + keep), so a
  budget error can't wipe staged invoices; roll back the optimistic user turn.
- stable attachment ids (no crypto.randomUUID — undefined over plain HTTP).
- editable Datum splatnosti + Popis fields; client-side extraction summary.

Security: DashProfile clipboard copy now uses an execCommand fallback (works
over plain HTTP) and only shows the success toast when the copy actually
succeeded — previously it falsely claimed to copy 2FA codes / the TOTP secret.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/schemas/ai.schema.ts

@@ -12,6 +12,18 @@ export const AiChatSchema = z.object({
     .max(100, "Příliš mnoho zpráv"),
 });
 
+export const AiHistoryAppendSchema = z.object({
+  messages: z
+    .array(
+      z.object({
+        role: z.enum(["user", "assistant"]),
+        content: z.string().min(1).max(8000),
+      }),
+    )
+    .min(1, "Žádné zprávy")
+    .max(20, "Příliš mnoho zpráv"),
+});
+
 export const AiBudgetSchema = z.object({
   budget_usd: z
     .union([z.number(), z.string()])